The decay of hot and rotating compound nucleus 164 Yb * , formed in heavy ion reaction 64 Ni+ 100 Mo at both below-and above-barrier energies, is studied on the basis of the dynamical cluster-decay model (DCM) with effects of deformations and orientations of nuclei included in it. There is only one parameter in this model, namely the neck-length parameter, which varies smoothly with the temperature of the compound nucleus at both below-and above-barrier energies, and its value remains within the range of validity of the proximity potential. The emission of light particles (xn, x-neutrons, x = 1-4) as well as the energetically favoured intermediate mass fragments (IMFs) of both the light (5 A 2 20) and heavy (40 A 2 50) masses, together with the symmetric fission (SF) channel ((A/2) ± 20), is considered as the dynamical collective mass motions of preformed fragments or clusters through the barrier. The light-mass IMFs, the heavy-mass fragments (HMFs) and SF, constituting the fusion-fission (ff) cross-section, contribute only at above-barrier energies, and are compatible with the CASCADE analysis of experimental data. A best fit to data is obtained for two different neck-length parameters, one for light particles (LPs) and another for all other decay channels, the ff crosssection. The barrier height corresponding to the neck-length parameter for LPs gives 'barrier lowering' in a straightforward way for the best-fitted fusionevaporation cross-sections in DCM, and, in contrast to the (statistical model) analysis of experimental data, results in largest contribution for 1n emission. A further study is called for both the LPs and ff channels.

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The role of deformations and orientations of nuclei is studied for the first time in cluster decays of various radioactive nuclei, particularly those decaying to doubly closed shell, spherical 208 Pb daughter nucleus. Also, the significance of using the correct Q-value of the decay process is pointed out. The model used is the preformed cluster model (In this model, cluster emission is treated as a tunneling of the confining interaction barrier by a cluster considered already preformed with a relative probability P 0 . Since both the scattering potential and potential energy surface due to the fragmentation process in the ground state of the parent nucleus change significantly with the inclusion of deformation and orientation effects, both the penetrability P and preformation probability P 0 of clusters change accordingly. The calculated decay half-lives for all the cluster decays investigated here are generally in good agreement with measured values for the calculation performed with quadrupole deformations β 2 alone and "optimum" orientations of cold elongated configurations. In some cases, particularly for 14 C decay of Ra nuclei, the inclusion of multipole deformations up to hexadecapole β 4 is found to be essential for a comparison with data. However, the available β 4 -values, particularly for nuclei in the mass region 16 A 26, need be used with caution.

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The dynamical cluster-decay model (DCM) is extended to a positive Q-value (Qout), heavy compound system 116Ba*, with complete angular momentum and charge dispersion effects included in it. The contributions due to both the light particles (LPs) and intermediate mass fragments (IMFs) are considered to give the total cross section. Interestingly, instead of the complete IMF spectrum observed for lighter systems such as 48Cr* and 56Ni*, here two small ‘windows of IMFs’ are predicted, one for light masses (2 ⩽ Z ⩽ 9) and another for the heavy mass end of symmetric and nearly symmetric fragments (14 ⩽ Z ⩽ 28), in agreement with the available data for the light mass ‘IMF window’ and its indications of possible extension to the heavier mass fragments. Within a non-statistical model description, the definition of phase space is found to be contained in the DCM definition of the ‘IMF window’ for the compound nucleus process. As in experiments, the calculated excitation functions are shown to put a limit on the minimum incident centre-of-mass energy required for the production of IMFs, and it will be of further interest to observe in experiments the predicted structures in the excitation functions of both the individual fragments, like for 12C decay, and the summed-up cross sections. Also, further measurements of the total kinetic energies of the fragments are called for.